1. Field of the Invention
The present invention relates to an apparatus for automatically starting up a nuclear reactor and more particularly to an apparatus for automatically manipulating a nuclear reactor until the temperature and the pressure reach the predetermined levels after the reactor has reached a critical state.
2. Description of the Prior Art
There are three essential steps in the starting-up operation of a nuclear reactor: critical manipulation, heat-up and pressurization manipulation, and power-up manipulation. These steps are performed in the order mentioned. The critical manipulation is defined as a step of gradually withdrawing the control rods when the reactor is in the resting state, until the reactivity .rho. of the reactor becomes unity. The reactivity of 1 means that the reactor is at the critical condition where nuclear fission in the reactor proceeds continuously and that the neutron flux to cause the following nuclear fission is constant. Namely, the reactor is said to be critical, subcritical and supercritical, respectively, when the reactivity is equal to, less and more than unity. Whereas the nuclear fission attenuates in the subcritical condition (.rho.&lt;1), the fission proceeds indefinitely in the supercritical condition (.rho.&gt;1) and the reactor may run away unless any safety control is performed. The critical manipulation is, in other words, to cause the nuclear reaction in the reactor to proceed from the nuclear resting state (where even if neutron flux is generated the resultant nuclear fission attenuates) to a state where the reactivity .rho. is 1 or slightly larger than 1, i.e. around 1.01, by gradually withdrawing the control rods.
Whether the reactor has got critical or not, is judged by checking the reactor period. The reactor period, expressed by the reciprocal 1/(dn/dt) of the rate dn/dt of change in the neutron flux n in the reactor with time t, is defined as the mean time required for the power level of the reactor to change by the factor e = 2.71828. The dimension of the reactor period is time. Namely, the period is infinite for the reactivity of 1, i.e. critical state, a negative finite value for the subcritical value, and a positive finite value for the supercritical state. The state of the reactor being critical can be identified by the fact that the reactor period assumes a positive value and the time during which the neutron flux is at a certain value lasts longer than, for example, a predetermined period. In such a critical state, the reactor thermal power is less than about 1% of the rated power.
After the reactor has become critical, the control rods are further withdrawn, while the level of the reactor water is kept constant, to subject the reactor to heat-up and pressurization. During this heat-up and pressurization process, the reactor is separated from a turbine and the like so that the supply of water into the reactor and the derivation of the steam are not normally carried out. Exceptionally, however, the decrement of a reactor water level caused by a reactor water purifying system is compensated by a water supply system, and some fractional quantity of the steam is taken out after the later stage of the heat-up and pressurization process, for the warming-up of the turbine.
The present invention provides an apparatus for automatically performing the manipulation of a reactor for heat-up and pressurization after the completion of the critical manipulation. For this purpose, the following problems must be solved.
First, for the heat-up and pressurization, the operator manually withdraws the control rods by monitoring various controlled variables so that the burden imposed on him is severe and the time required for manipulation widely varies depending upon the skill of operator. Secondly, it is specifically necessary in this stage to keep constant the heat-up ratio of the temperature in the reactor so as not to expose the pressure vessel to thermal impact. Thirdly, the reactivity in the heat-up and pressurization process is derived by summing the positive reactivity effect due to the withdrawal of the control rods and the negative reactivity effect due to the temperature in the reactor, but since the temperature varies, it is necessary for keeping the heat-up ratio constant to control the withdrawal of the control rods so as to compensate for the variation. Fourthly, since the negative reactivity effect varies non-linearly with the temperature in the reactor, it is difficult to compensate for the reactivity effect. Fifthly, the response to the variation of the reactor temperature caused due to the withdrawal of the control rods, is slow. Sixthly, since the pressure in the boiling water reactor has an influence on the reactor temperature, the reactivity should be corrected with respect to pressure.
The reactor thermal power now after the completion of the heat-up and pressurization manipulation is about 10% of the rated power and thereafter the steam is conducted to the turbine to increase the speed thereof while the power-up manipulation is performed by controlling the control rods and the recirculation flow.
The term "reactor temperature" used in this specification refers to the temperature of the water serving as coolant and moderator in the reactor or of the wall of the pressure vessel. Although in the start-up operation of a reactor the heat-up ratio of the temperature of the reactor water must be actually kept constant, it is difficult to measure the temperature exactly. Instead, the temperature at the wall of the pressure vessel may be conveniently used as the measure of the heat-up ratio. Accordingly, in this specification, the reactor temperature thus defined is used to represent the temperature of each portion useful to monitor the heat-up ratio of the reactor.